Standard compact discs have a diameter of about 12 cm; they contain audio information and auxiliary information, for example relating to the number of the music titles to be found on one CD, identifying the composer, the artists, reproduction duration, and the like; additionally, data relating to synchronization and error correction for proper operation of the CD player may be stored. The data typically are stored digitally in pulse code modulation technology. The 1/0 pulses of a pulse code modulation (PCM) signal is formed in the plate as a microscopically small depression or pit. The pits are arranged in a spiral, impressed on the CD, extending from an inward point outwardly toward the circumference. The tracks, radially, have a tracking distance or spacing of about 1.6 micrometers. A large number of such tracks, measured in radial direction, are located adjacent each other. The digital data are defined by the pit length and the spacing between pits, so that the data used in the CD can provide the eight-to-fourteen modulation (EFM).
The spiral has approximately 13,000 information tracks measured in radial direction. The spiral is impressed in a mirrored metallic surface formed as an information plane of the CD. On the impression side, a protective letter coating is applied thereover on which the label of the CD is attached. At the other side, the information plane is covered with a transparent cover layer of precisely predetermined index of refraction. The information on the CD is scanned through this transparent cover layer by a laser beam. Scanning is carried out with a constant bit rate. The speed of the plate drive motor is changed as the scanning radius changes, from about 500 rpm when an inner spiral is tracked, to about 200 rpm towards the edge of the CD. The scanned information is derived as digital data which is first recorded in a 16 kBit random access memory (RAM) of the signal processing device, to be read out therefrom at a clock frequency which is precisely quartz controlled. Speed changes of the CD drive motor thus will not have an influence on synchronization of reproduced output with respect to recorded input.
The CD is scanned in the CD player by a laser diode having a light output power of about 1 mW, and a wave length which is in the infrared region. The laser beam is focussed by a movable optical scanning system, movable in two directions--axially and radially with respect to the CD--to scan the tracks in the information plane of the CD. A focus servo loop circuit for precise focussing is provided, as well as a tracking and a track control circuit for precise tracking of the scanning optic on the signal track being scanned. The scanning optics is controlled by a course control, by securing the scanning optic to a slider which is radially movable with respect to the rotating plate by a motor drive. A slider or carriage control loop is provided to control the respective radial shift of the slider or carriage so that the scanning optic can pass or scan radially across the entire radius of the CD. In addition, a fine control is provided. At any position of the slider, the track control circuit of the scanning optic can control the scanning optic over a fine tracking range of about 40 adjacent tracks at any position of the slider or carriage.
The scanning values or signals derived from the scanning optic are amplified and converted into an analog output signal to be applied to a signal processing device. The analog output signals are then transformed into a digital signal synchronized with the clock frequency of the signal processing apparatus. Thereafter, the EFM demodulation and a subcode demodulation are carried out. The subcode demodulation results in a subcode Q-signal which includes a plurality of specified bits, namely
______________________________________ synchronizing bits control bits monitoring bits 72 data Q bits having 9 Q-data: TNO: number of the music piece X: characteristic of the chopping rate MIN SEC FRAME data regarding playing time and ZERO title in binary coded decimal AMIN (BCD) code ASEC AFRAME ______________________________________
As the playing time of the CD increases, the time indication also increases. In the pauses between music titles, the time indication is reset back to zero. The data derived from EFM are recorded in the RAM. The recorded data are decoded by CIRC decoding, and the decoded data are then read out and applied via an error correction circuit and a digital filter to a digital-analog (D/A) converter. The analog output signal is separated over a quieting or muting circuit, and applied to respective left and right (L and R) stereo channels of an audio component of a receiver or audio amplifier.
Operation of CD players in automotive vehicles unavoidably results in tracking jumps due to mechanical shocks and vibrations to which the CD player will be subjected. The scanning optic, for example, upon being subjected to a shock, may lose the then scanned track and, after the shock has dissipated, may be positioned, at random, opposite another track. To reduce shocks and resistance to vibration to the CD player, it has been proposed to suspend the mechanical portion of the CD player, including the drive motor and the scanning optic, with the slide, in a vibration and shock damped suspension. This substantially increases the volume of the CD player; complete isolation from shock and vibration due to road conditions, for example, cannot be avoided; further, the limited space within the dashboard area of a vehicle usually prevents elaborate shock and vibration resistant suspension systems.